The present invention relates to a method for reducing emissions, such as sulfur oxides (SO.sub.2), nitrous oxides (N.sub.2 O) and carbon monoxide (CO), to the atmosphere from the combustion of nitrogen containing combustible compounds. More specifically, this invention relates to a method for reducing such emissions when combusting solid fuels or the like in fluidized bed combustors.
Fluidized bed combustion (FBC) is known to result in efficient combustion, as well as, efficient sulfur oxide and nitrogen oxide emission control. Due to intimate mixing of solid material and gases an efficient combustion is achieved in the fluidized bed already at low combustion temperatures 700.degree.-1000.degree. C. Sulfur oxides (SO.sub.2) are captured at this relatively low combustion temperature, which is optimal for SO.sub.2 reduction by milled limestone injected with the fuel into the combustion chamber. The relatively low combustion temperature needed in a FBC also results in reduced formation of nitrogen oxides NO.sub.x, i.e. NO.sub.2 and NO. NO.sub.x emissions from FBC are typically in the range of 100-400 ppm.
The above mentioned improvements in fluidized bed technology over conventional flame combustion are enhanced in circulating fluidized bed combustion (CFBC). Besides providing the possibility of burning different fuels in the same combustor, i.e. both high and low grade fuels, the CFB boiler technology provides better means of controlling the combustion process leading to improved boiler efficiency and improved control of sulfur oxide (SO.sub.2) and nitrogen oxide (NO.sub.x) emissions. NO.sub.x emissions from CFB boilers are in the range of 50-200 ppm.
Recently attention has been focused on the emission of nitrous oxide (N.sub.2 O) from combustors. The atmospheric concentration of N.sub.2 O increases constantly and it is believed to have an effect on the atmosphere. While the greenhouse effect has mainly been associated with increased CO.sub.2 levels in the atmosphere, concern is now growing about strong infrared absorbers, such as N.sub.2 O, contributing to the greenhouse effect even if the concentration of N.sub.2 O is much lower than that of CO.sub.2. Further, according to recent research, N.sub.2 O may indirectly adversely affect the stratospheric ozone layer as well.
Recent studies indicate that fluidized bed combustion, while achieving significantly lower levels of NO.sub.x emissions compared to flame or pulverized coal combustion, may yield higher levels of N.sub.2 O. It has been reported that N.sub.2 O emissions are generated in higher degree in combustors with low combustion temperatures such as 750.degree.-900.degree. C. At higher temperatures the formation of N.sub.2 O does not seem to be a problem, as the formation of N.sub.2 O is minor, while the reduction of N.sub.2 O to N.sub.2 at the same temperature is high.
The likely main mechanism for N.sub.2 O formation from fuel nitrogen has been suggested to be the following:
FUEL-N.fwdarw.HCN PA1 HCN+O.fwdarw.NCO PA1 NCO+NO.fwdarw.N.sub.2 O PA1 (a) Supplying nitrogen containing fuel and an oxygen containing as for combustion of the fuel into the combustion stage of the combustor to produce flue gases which contain particles. PA1 (b) Maintaining a temperature of about 700.degree.-1000.degree. C. in the combustion stage. PA1 (c) Supplying a Ca-based sulfur absorbing sorbent to the combustor for reducing sulfur emissions in the flue gases. PA1 (d) Without separate removal of the particles from the flue gases affecting vigorous and intimate mixing of the particles and flue gases by increasing the velocity of the flue gases before leaving the combustion stage, and then decreasing the velocity from the increased level. PA1 (e) Introducing an N.sub.2 O decomposing catalyst into the flue gases and particles immediately after increasing and decreasing of the velocity thereof. And, PA1 (f) mixing the N.sub.2 O decomposing catalyst with the flue gases and particles to effect decomposition of the N.sub.2 O. And, PA1 (g) discharging the flue gases with particles and decomposed N.sub.2 O therein from the combustor. PA1 (a) Supplying nitrogen containing fuel and an oxygen containing gas for combustion of the fuel into the combustion stage of the combustor to produce flue gases which contain particles. PA1 (b) Maintaining a temperature of about 700.degree.-1000.degree. C. in the combustion stage. PA1 (c) Supplying a Ca-based sulfur absorbing sorbent to the combustor for reducing sulfur emissions in the flue gases. PA1 (d) Introducing a Ca-based sorbent N.sub.2 O decomposing catalyst to effect decomposition of the N.sub.2 O into the flue gases and particles and intimately mixing it with the flue gases, while maintaining temperature high enough to produce calcined sorbent. PA1 (e) Discharging the flue gases with particles, decomposed N.sub.2 O, and calcined sorbent there from the combustor; And, PA1 (f) removing the calcined sorbent from the discharged flue gases and recirculating it to provide part of the sorbent introduced in step (c). PA1 (a) Supplying nitrogen containing fuel and an oxygen containing gas for combustion of the fuel into the combustion stage of the combustor to produce flue gases which contain particles. PA1 (b) Maintaining a temperature of about 700.degree.-1000.degree. in the combustion stage. PA1 (c) Supplying a Ca-based sulfur absorbing sorbent entrained in air to the combustor for reducing sulfur emissions in the flue gases. PA1 (d) Passing the flue gases through a constriction in the combustor. PA1 (e) Introducing an N.sub.2 O decomposing catalyst into the flue gases immediately after they pass through the constriction, to effect mixing of the catalyst with the flue gases and decomposition of the N.sub.2 O. And, PA1 (f) discharging the flue gases with decomposed N.sub.2 O therein from the combustor. PA1 An upright vessel having a bottom portion, a top portion, and a central portion, with cross-sectional areas. Means for introducing fuel and oxygen containing gas into the bottom portion of the upright vessel, and for maintaining a fluidized bed therein. Means for discharging flue gases from the top portion of the upright vessel. Means defining a constriction in the cross-sectional area of the vessel between the top and bottom portions thereof through which flue gases flow, the constriction being small enough to significantly increase the velocity of flue gases flowing therethrough. And, means for introducing catalyst into the vessel in the top portion thereof, above the constriction. PA1 The construction is simple, with one fluidized bed combustor/reactor only. PA1 The mixing chamber forms a simple continuation of the combustion chamber. PA1 The same water tubes (water panels) can be used for forming the combustion chamber and the mixing chamber, whereby water circulation in the steam/water system is easy to arrange. PA1 The single fluidized bed combustor arrangement does not need as much space as a system having two interconnected fluidized bed systems, both needing separate fluidizing means, particle separators and supports; and PA1 The total height of the present system and the height needed for the building housing it are lower than in a system including two separate fluidized be systems.
At the present time, however, the details of the mechanisms of N.sub.2 O formation are not known.
The combustion temperature and the type of fuel seem to be the main factors affecting the N.sub.2 O emission. According to tests the emissions decrease significantly when the combustion temperature is increased over 900.degree. C. In the combustion of coal, N.sub.2 O emissions varied typically from 30 to 120 ppmv (3% O.sub.2, dry), whereas in the combustion of oil shale, peat and wood waste N.sub.2 O emissions were typically significantly lower, below 50 ppmv.
There seems to be a strong correlation between temperature and both NO.sub.x and N.sub.2 O emissions. Changes to the combustion operating parameters affect NO.sub.x and N.sub.2 O emissions inversely. Increasing temperatures result in higher NO.sub.x and lower N.sub.2 O. Weaker correlations appear to exist for other parameters. A bed temperature increase in the combustion chamber would however result in reduced capability to capture SO.sub.2. Staged combustion seems to reduce both N.sub.2 O and NO.sub.x emissions to a certain degree, but easily leads to an increase in carbon monoxide (CO) concentration.
One method to reduce the N.sub.2 O emissions, suggested in U.S. Pat. No. 5,043,150, is to add hydrogen radicals to the flue gases by providing an additive capable of forming hydrogen radicals at temperatures equal to or higher than those of the flue gases. The hydrogen radicals effectively destroy N.sub.2 O through the homogenous gas reaction EQU (A) N.sub.2 O+H.fwdarw.N.sub.2 +OH
Additives providing hydrogen radicals are e.g. methane, liquified petroleum gas, oil, alcohol, pyrolyser gas, or gasifier gas. The hydrogen radical formation is favored at higher temperatures. Apparently by increasing the flue gas temperature the rate of the reaction (A) is also increased and a rapid N.sub.2 O destruction may be accomplished.
U.S. Pat. No. 5,048,432, European patent application EP 0 406 185, and German patent application DE 39 33 286 all suggest raising the temperature of flue gases to a level above 900.degree. C. for reducing N.sub.2 O emissions.
Other parameters potentially affecting N.sub.2 O emissions have also been studied, such as increase of excess air, injection of ammonia, recirculation of fly ash, CO concentration, and addition of limestone. Some studies show slight effects of above mentioned parameters, either decreasing or increasing N.sub.2 O emissions, but no clear picture has developed. E.g. N.sub.2 O has been found to decompose on the surface of calcined limestone CaO, while simultaneously the NO emissions increase. It has, on the other hand, also been reported that N.sub.2 O may result from NO reduction on CaSO.sub.x surfaces, CaSO.sub.x being formed by reduction of SO.sub.2 with CaO, the higher the Ca/S ratio the higher the NO reduction on CaSO.sub.4. Therefore, until now, no clear conclusion could be drawn on the effect of boiler limestone addition on emissions.
It is, however, known that N.sub.2 O emissions from fluidized bed boilers may be on the level of 50-200 ppm, i.e. higher than desired. Therefore, according to this invention a method is provided for reducing the emissions of N.sub.2 O from conventional fluidized bed boilers and circulating fluidized bed boilers, atmospheric or pressurized. The method according to the invention also may decompose CO in the flue gases, and improve the SO.sub.2 reduction in flue gases from a fluidized bed boiler.
The method of the invention simultaneously reduces N.sub.2 O, SO.sub.2, and CO in flue gases, thereby improving the environmental properties of fluidized bed combustor systems.
In the parent application the N.sub.2 O emissions were reduced by providing reactor, typically in the form of a second fluidized bed, removing the particles from the flue gases from the first fluidized bed, and then introducing a N.sub.2 O decomposing catalyst into the flue gases without significantly raising the temperature of the flue gases. The catalyst was selected from the group consisting essentially of calcium based sorbents, siderite, ankerite, NiO, CuO, and Mgo.